There is a vehicle communication system in which an operation of a target mounted on a vehicle such as locking or unlocking of doors of the vehicle is permitted or prohibited based on a radio signal transmitted or received between a vehicle-mounted device mounted on the vehicle and a portable device carried by a user of the vehicle. This vehicle communication system is provided to improve security in the vehicle such as an automobile and convenience of the user.
The vehicle-mounted device and the portable device respectively include a transmission unit and a reception unit which transmit and receive a radio signal. The transmission unit of the vehicle-mounted device transmits a response request signal outside or inside of the vehicle. When the portable device approaches the vehicle up to a communicable distance, the reception unit of the portable device receives a response request signal transmitted by the transmission unit of the vehicle-mounted device and the transmission unit of the portable device returns a response signal. When the reception unit of the vehicle-mounted device receives the response signal, the vehicle-mounted device authenticates the portable device using an ID (identification information) included in the response signal, and when the authentication succeeds, the vehicle-mounted device locks or unlocks doors of the vehicle.
However, a response request signal transmitted from a vehicle-mounted control device is relayed by a repeater and is received by a portable device in the distance, a wrong communication act of disguising as if the portable device is in a periphery of the vehicle may be performed. The wrong communication act using the repeater is called a relay attack. By the relay attack, a malicious third party who is not an owner of the vehicle may commit a crime such as theft by unlocking a door of the vehicle or starting an engine.
Therefore, as an anti-crime measure against the relay attack, for example, in JP-A-2012-144905, two radio signals having different signal intensities are transmitted from the vehicle-mounted device and reception intensities of both of the radio signals are measured by the portable device. In a case where a difference between the reception intensities of the two radio signals is larger than a threshold value, control of a target is permitted. In a case where the difference between the reception intensities of the two radio signals is equal to or smaller than the threshold value, it is determined that there is a wrong communication using a repeater and the control of the target is prohibited. In the repeater used for the relay attack, the two radio signals transmitted from the vehicle-mounted device are relayed, but intensities of the radio signals cannot be reproduced. For this reason, in a case where the relay attack is performed, a difference between the reception intensities of the two radio signals measured by the portable device does not occur and the difference is equal to or smaller than a threshold value, and thus it is determined that a wrong communication using the repeater and the control of the target can be prohibited.
However, when the two radio signals having different intensities are transmitted from the vehicle-mounted device, if the portable device moves, a difference occurs in the reception intensities of the two radio signals measured by the portable device despite the relay attack being performed and security is decreased. In JP-A-2012-144905, as a measure, a transmission time interval of the two radio signals from the vehicle-mounted device is set so that the difference between the intensities of the two radio signals is equal to or smaller than the threshold value.
In addition, the reception intensity of the radio signal measured by the portable device becomes larger as the portable device approaches the vehicle-mounted device, but the reception intensity is saturated at a certain value. That is, since a reception intensity which can be measured by the portable device has an upper limit, when the portable device approaches the vehicle-mounted device close to more than a predetermined distance, even if the two radio signals having different intensities are transmitted from the vehicle-mounted device, but a difference does not occur in the reception intensities of the two radio signals measured by the portable device. For this reason, the control of the target is not permitted and convenience of a user is decreased.
On the other hand, in JP-A-2012-167446, a first measurement signal of which an intensity is “large” and a second measurement signal of which an intensity is “medium” are transmitted from the vehicle-mounted device. In a case where a reception intensity difference between both of the measurement signals measured by the portable device is equal to or smaller than a threshold value, the first measurement signal and a third measurement signal of which an intensity is “small” are transmitted from the vehicle-mounted device. By setting the intensity of the third measurement signal smaller than an upper limit of a reception intensity measurable by the portable device, a difference certainly occurs in the reception intensities of the first measurement signal and the third measurement signal.